1. Field of the Invention
The invention relates to a pump for pumping a fluid comprising a liquefied gas.
Such a pump, defined as a cryogenic pump, can be used, for example, for pumping a cryogenic fluid which at least partly comprises at least one liquefied gas--such as liquefied nitrogen, oxygen, hydrogen or argon or liquefied air--and may also comprise gaseous fluid. The pump may also be used for pumping liquefied butane or propane. The pump can be formed, for example, for pumping fluid from a reservoir into a container or any apparatus and for increasing the pressure of the fluid from a value of, for example, at most 1.5 MPa to, for example, 15 to 50 MPa.
2. Description of the Prior Art
A pump disclosed in U.S. Pat. No. 4,915,602 has a drive device with a support and a pump housing. This has an elongated, sleeve-like bearer, which is fixed to the support, and a bush which is partly inserted into said bearer and is closed at its end projecting from the bearer by an end member. A piston which is displaceable in the bush is connected to a movable part of the drive device by a piston rod. The inner surface of the sleeve-like bearer has annular grooves into which seals for sealing the piston rod fit. The largest part of the bearer, the bush and the end member are present in a container whose wall has a shell connected tightly and firmly in the vicinity of the drive device to the bearer and a shell which can be detached by means of screws and is connected by means of said bolts. The two shells have an inner wall, an outer wall and an evacuated intermediate space in between. The inlet of the pump is connected to a pump chamber which is present in the bush and adjacent to the end face of the piston by the inner space of the container and two flow paths each having a non-return valve. The pump chamber is furthermore connected to a primary outlet via a non-return valve. The container has a secondary outlet through which vaporized fluid flows out of the container during operation.
This pump has proved satisfactory per se. However, the production costs of the pump are increased by the production and assembly of the wall of the container containing the major part of the sleeve-like bearer and bush, which wall consists of a plurality of parts and has evacuated intermediate spaces. In addition, the maintenance of the known pump gives rise to a relatively large amount of work and long down times. If, for example, the bush and the piston are to be cleaned or one of the seals attached to the piston or in the sleeve-like bearer or a part of a non-return valve are to be replaced, it is necessary, inter alia, to separate the two shells of the container from one another and then connect them together again and to slacken a number of bolts and nuts and screw them together again afterwards.
The pump disclosed in German Examined Patent Application 1,169,973 and intended for pumping a liquefied gas possesses an elongated, hollow bearer, a bush present partly in the inner space of the bearer and displaceable piston. The bush has two collars adjacent to the inner surface of the bearer and, between said collars, is separated from the inner surface of the bearer by an intermediate space. A reservoir is connected via a feed pipe to the inlet opening into the intermediate space. The bush contains a pump chamber which is adjacent to the end face of the piston and is connected to a primary outlet of the pump via a non-return valve. The bearer is provided with a secondary outlet which is connected to the intermediate space and is connected via a return pipe to the reservoir. The piston is hollow and has, in its end face, an orifice which can be closed by means of an axially displaceable closure member and forms a valve together with the latter. The stated intermediate space is connected by radial holes in the wall of the bush and by elongated holes in the wall of the hollow piston to the cavity present therein. During operation of this pump, fluid flows from the reservoir through the feed pipe and the inlet into the stated intermediate space and from this into the cavity of the piston. If the piston is moved back and forth, fluid can pass from the cavity of the piston into the pump chamber and can be transported by the piston from there to the primary outlet. If the fluid present in the intermediate space is heated, it expands, fluid also vaporizing. Fluid can then flow from the intermediate space through the secondary outlet and the return pipe back into the reservoir.
In this known pump, that section of the bearer which contains the bush has an external diameter which is large in comparison with the diameters of the bush and of the piston, and a large outer surface adjacent to the environment of the pump. Furthermore, that section of the bush which contains the pump chamber projects from the bearer. Thus, a large amount of heat can flow from the environment of the pump through the bearer and the bush end section projecting from said bearer into the bearer and the bush. The cryogenic fluid is thus greatly heated so that a large amount of fluid vaporizes. Furthermore, the secondary outlet is in the same longitudinal section of the bearer as the inlet and is connected to this at the highest point of the intermediate space. Furthermore, the cross-sectional area of the intermediate space is very much larger than the cross-sectional area of the inlet passage. There is therefore no defined fluid flow in the intermediate space. The intermediate space can in certain circumstance therefore contain relatively warm liquid or gas bubbles at different points, and this warm liquid and these gas bubbles may remain for a long time in the intermediate space. In addition, the liquid flowing through the holes of the bush into the bush and the cavity of the piston is pushed back and forth during displacement of the piston, but the volume of the cavity remains constant and is permanently relatively large. Gas bubbles may therefore also enter the cavity of the piston and remain therein for a relatively long time. However, gas bubbles present in the intermediate space and in the cavity of the piston reduce the efficiency of the pump and may even cause breakaway of the liquid flow. This known pump therefore does not function reliably and in particular is scarcely suitable for continuous operation over long periods. The operational reliability of the pump according to German Examined Patent Application 1,169,973 could be improved at most by thermally insulating from the environment the bearer and the bush section projecting from it by means of an additional insulation. However, such an additional insulation is not disclosed and would furthermore probably have disadvantages similar to those of the shells of the pump disclosed in U.S. Pat. No. 4,915,602.
The reservoir frequently also contains solid, fine impurity particles which are dispersed in liquefied gas and, for example, consist of a carbon compound and, together with the liquefied gas, may enter the bearer, the bush and the cavity of the piston. Since no defined, strong flow occurs in the intermediate space of the bearer and also in relatively large parts of the cavity of the piston of the pump according to German Examined Patent Application 1,169,973, impurity particles may accumulate in the interior of the bearer and in the piston, so that the pump frequently has to be dismantled and cleaned.
The piston rod which serves for moving the piston must be sealed somewhere. German Examined Patent Application 1,169,973 does not disclose any seals which serve this purpose. However, it appears probable that the changing of these seals which is required in practice usually after a certain number of operating hours necessitates a relatively long down time, similarly to the pump described above and disclosed in U.S. Pat. No. 4,915,602.